1. Field of the Invention
This invention relates to novel sliding mechanisms, and more particularly to a sliding mechanism for nuclear reactors including pins and rollers for guiding a control rod for use in a reactor core.
2. Description of the Prior Art
A control rod for use in a core of a nuclear reactor has guide means including rollers, formed of a cobalt base superalloy, and pins formed of a forged cobalt base alloy. The cobalt base superalloy used for forming the rollers contains 1% carbon and 28% chromium. Because of this composition, the superalloy contains large amounts of carbides and has high hardness, and it is highly resistant to corrosion with respect to pure water at high temperature and high pressure due to the high chromium content. These properties make the superalloy suitable as material for forming the rollers of a control rod. On the other hand, the forged cobalt base alloy used for forming the pins has a relatively low carbon content of 0.1% and a low chromium content of 20% because the alloy is required to be forgeable, workable and weldable. Nevertheless, the alloy has high wear and corrosion resistance.
Due to the fact that such alloys contain about 65% of cobalt in roller material and about 50% of cobalt in pin material, abrasion powder of cobalt is produced by the wear of the two members while in service, and cobalt is released from the alloy due to corrosion by pure water at high temperature and high pressure. This raises the problem that the cobalt ion and crud in the coolant becomes radioactive and circulates through the system.
In Toshiba Review, Vol. 34, No. 10 (1979) the use a nitronic #60 alloy is proposed. This alloy consists essentially of, by weight, 0.07% carbon, 8.48% manganese, 4.03% silicon, 16.94% chromium, 8.58% nickel, 0.25% molybdenum and the balance iron and having a Rockwell C scale hardness HRC 22 and a Vickers hardness Hv 253 for forming the pins and CFA consisting essentially of, by weight, 0.57% manganese, 0.27% silicon, 37.5% chromium, 3.72% aluminum, 1.12% molybdenum and the balance nickel and having a Rockwell C scale hardness H.sub.RC 55 and a Vickers hardness Hv 595 for forming the rollers, to obviate the problem of the radioactive cobalt. It has been found that this combination poses the problem that since the roller and the pin have differences in hardness of 33 in H.sub.RC and 342 in Hv, wear would be caused particularly on the pin which is lower in hardness.